Lord of the White Tree

Throughout the history of Númenor, its people had tended a White Tree that grew in the royal courts of its Kings in their city of Armenelos. That tree, named Nimloth, had been a gift of the Eldar, and was a descendant of a tree made by Yavanna herself as an image of Telperion of the Two Trees. It was maintained with reverence until the time when Sauron corrupted the last King, and made clear his purpose to destroy the White Tree of Númenor.

This indeed Sauron did, but not before Isildur had recovered a fruit from the White Tree Nimloth, which grew into a sapling. When Isildur came to Middle-earth after the Downfall, he planted that young White Tree in the courts of his city of Minas Ithil. Once again, however, the White Tree was destroyed by Sauron when he attacked and captured Isildur's fortress, and once again a seedling was saved, which would later give rise to the line of the White Trees of Minas Tirith.

Aragorn was a descendant of the House of Isildur through many generations, and so it was fitting that his emblem should be a White Tree. The symbol not only recalled the deeds of his ancestor, but also the White Tree that had stood in Minas Tirith during the time of its Kings, reflecting Aragorn's own claim to the Kingship of Gondor. Thus Arwen created a banner for Aragorn carrying the White Tree, a banner that he flew as he entered the Battle of the Pelennor Fields.

There were thus many connections between Aragorn and the White Tree - and, further, he recovered another seedling from the snows of Mindolluin, and thus reinstated the line that had been lost centuries beforehand. It was natural, therefore, for Legolas to name Aragorn as the 'Lord of the White Tree', though it is less clear whether this was any kind of formal title. Indeed, our only recorded use of this title cames at a time when Legolas was speaking of his own people, the forest-dwelling Folk of the Wood, and so it may be that the image of a tree came naturally to his mind in that moment, rather than being a more regular form of address.